14. Why Humans Walk On Two Legs (A级)

（替换2007年用书理工类阅读理解第四十九篇：Retinal Prosthesis Helps the Blind Regain Eyesight）

A team of scientists that studied chimpanzees trained to use treadmills has gathered new evidence suggesting that our earliest apelike ancestors started walking on two legs because it required less energy than getting around on all fours.

"When our earliest ancestors started walking on two legs, they took the first steps toward becoming human," said lead researcher Michael Sockol of UC Davis1. "Our findings help answer why." Sockol worked for two years to find an animal trainer willing to coax adult chimps to walk on two legs and to "knucklewalk" on all fours on the sort of treadmill found in most gyms.

The five chimps also wore face masks used to help the researchers measure oxygen consumption. While the chimps worked out, the scientists collected metabolic and other data that allowed them to calculate which method of locomotion used less energy and why. The team gathered the same information for four adult humans walking on a treadmill.

The researchers found that human walking used about 75 percent less energy and burned 75 percent fewer calories than quadrupedal and bipedal2 walking in chimpanzees. They also found that for some but not all of the chimps, walking on two legs was no more costly than knucklewalking.

"We were prepared to find that all of the chimps used more energy walking on two legs - but that finding wouldn't have been as interesting," Sockol said. "What we found was much more telling. For three chimps, bipedalism3 was more expensive, but for the other two chimps, this wasn't the case. One spent about the same energy walking on two legs as on four. The other used less energy walking upright." These two chimps had different gaits and anatomy than their knucklewalking peers.

Taken together, the findings provide support for the hypothesis that anatomical differences affecting gait existed among our earliest apelike ancestors, and that these differences provided the genetic variation which natural selection could act on when changes in the environment gave bipeds an advantage over quadrupeds.

Fossil and molecular evidence suggests the earliest ancestors of the human family lived in forested areas in equatorial Africa in the late Miocene era some 8 to 10 million years ago, when changes in climate may have increased in distance between food patches. That would have forced our earliest ancestors to travel longer distances on the ground and favored those who could cover more ground using less energy.

"This isn't the complete answer," Sockol said. "But it's a good piece of a puzzle humans have always wondered about: How and why did we become human? And why do we alone walk on two legs?" (435 words)

词汇：

chimpanzee n. 黑猩猩（又作chimp）

calorie n. 卡（路里），小卡

biped n 二足动物

treadmill n. 踏轮；踏车；单调工作

gait n. 步态，步法

coax v. 哄，哄劝

anatomic adj. 解剖（学）的

knucklewalk v. 用膝关节走路

quadruped n. 四足动物

metabolic adj.（新陈）代谢的

Miocene n.& adj. 中新世（的）

locomotion n. 运动（力）